1. Field of the Invention
The present invention relates to an optical pickup of a recording and reproducing apparatus for an optical data-recording medium such as an optical disk, card or the like.
2. Description of the Related Art
In recent years, there has been developed a high capacity and high recording density information recording medium referred to as a DVD (Digital Versatile Disc) as well as a system using the same medium. A DVD having a single recording layer on one side of the disk has a capacity of 4.7 GB. A two-layer DVD, which is compliant with the DVD standard, has been developed to increase the recording capacity.
There has been proposed an optical system having a high numerical aperture (high NA) and a shorter wavelength light-source in the optical pickup to be used for the next generation multi-layer optical disk system utilizing an optical disk having three or four recording layers in order to further increase the recording capacity. In such a multi-layer optical disk in which a plurality of recording surfaces and interposed spacer layers are alternately stacked, the focal point (the focused point or optimum converging position) of a light beam should be set to a surface of a desired recording layer. In other words, a converged light spot has to be irradiated upon the desired recording layer in order to read information from one side of the optical disk using the optical pickup.
In a converging optical system with a large effective NA, there occurs a large wave aberration (primarily spherical aberration). The amount of wave aberration is proportional to the numerical aperture to the fourth power. The wave aberration is caused by a thickness error of a transparent cover layer in the optical disk. The wave aberration is also caused by a fluctuation of the combined thickness (depth) of the desired recording layer (also referred to merely as a “thickness error”), which depth includes light-transmitting layers all the way from the cover layer to the recording layer in the case of multi-layer structure.
A spherical aberration brought about by the thickness error greatly widens the spot diameter of a light beam illuminated onto the desired recording layer. Thus the focal error signal of the optical pickup is suffered serious damage from the spherical aberration in a high NA irradiating optical system during the operation of focal servo system thereof, even if the optical system and focal servo system is optimized for the predetermined light-transmitting layers. As a result, a large thickness error caused by the light-transmitting layers on the recording layer moves the converged spot away from the optimized position so as to deteriorate the quality of the focal error.
Furthermore, it is necessary to mount a mechanism capable of correcting such spherical aberration on the optical pickup even if it has an optical system with a focal servo system that is optimized for predetermined light-transmitting layers. According to an earlier proposal aimed at compensating for such spherical aberration (Japanese Patent Application Laid-open Kokai No. H10-106012), the passing light beam is afforded a phase difference as a result of the fact that a liquid-crystal element is disposed in the optical path of the converging optical system of an optical pickup, and a voltage proportional to the detected spherical aberration is applied to the liquid-crystal element.
A method has also been proposed (Japanese Patent Application Laid-open Kokai No. 2000-171346) for compensating for such spherical aberration. In such a method, a focal servo function is performed using the light beam components in the vicinity of the optical axis of a light beam passing through an objective lens. Namely, spherical aberration is detected using the beam components on the outside thereof in order to obtain a signal that can be used to compensate for the spherical aberration.
In the conventional case that a part of the light beam passing through the objective lens is used to detect spherical aberration, the error signal of the focal point is greatly affected by the spherical aberration during the focal servo operation in a high NA converging optical system. Specifically, a large thickness error generated by the light-transmitting layer on a recording layer sometimes deflects the optimum converging position of the spot and degrades the focal error signal. For this reason, conventional optical pickup devices are still affected by spherical aberration, making it impossible to obtain adequate reproduction characteristics.